Published U.S. Patent Application 2005/0196872, Ser. No. 11/043,510, entitled “Mechanical Device For Mixing A Fluid Sample With A Treatment Solution” describes a two part device for mixing a blood sample with a treatment solution prior to release of the treated sample into a fluid analyte meter. A disadvantage of this system is that it is difficult to control pressurization in the device such that a correct sample volume is delivered to the test cartridge. As will be shown, the present invention overcomes this disadvantage.